AUTHOR:DAVID GLADWIN
TECHNICALLY SPEAKING
Although not to scale this drawing shows the effect of skin friction on the boundary layer velocity.The vortex generator creates a vortex which extends into the airflow clear of the
boundary layer which has the effect of adding energy to the sluggish boundary layer.VORTEX GENERATORS
Over the last 12 years or so I have built and
flown four BAe Hawks, one the original
Skymaster model, two of the Airworld 1:4.5
examples, and the big, 1:3.75 Skygate Hawk;
all have flown well and without vices, and inde-
ed the latter three are still fully serviceable and
have clocked up over three hundred flights.
Despite the benign handling of my fleet, I
have heard a few rumours of “quirky” han-
dling on some model Hawks, in direct con-
trast to my own models, which all show clas-
sic behaviour at the stall, with a gentle nose
drop at the point of stall with no tendency to
depart from controlled flight, both with and
without power.
So, why are my jets - and others, so well
behaved yet a few others have encountered
problems? The possible answer is to impro-
ve the aerodynamics with the wing “dres-
sings” i.e. small physical additions to the
wing, and on the basic Hawk there are three
such additions, plus one subtraction, that
being on the flap vane.
I often see Hawk models without the vortex
generators which are a feature of the Hawk
Mk 1, or have VGs which are not properly
angled, and therefore serve no purpose. The
airflow over any wing, indeed any surface, is
such that it has a boundary layer, up to about
1/ 4 inch deep on a fullsize machine, such
that airflow nearest the surface, i.e. in that
boundary layer, is slowed by skin friction. In
addition, as the airflow moves aft from the
point of maximum camber, it is moving from
low to high pressure, against the gradient,
and this can cause sluggish movement of the
boundary layer, leading to premature separa-
tion and possible premature stalling of that
section of the wing, and an increase in drag.
The function of the vortex generators is to
cause small vortices which extend up into the
free stream and capture the energy from the
freer flowing air and reenergize the sluggish
boundary layer, delaying separation, howe-
ver to create these vortices the little genera-
tors need to be set at an angle of about 15
degrees, a lesser or zero angle will not gene-
rate the vortex and is therefore of zero use. It
is true that the VGs themselves cause drag,
being set at this angle, but the overall effect
on the aircraft is advantageous, both in total
drag reduction and improved handling.
Two military aircraft on which VGs are clear-
ly seen are, as mentioned, the Hawk and also
the Harrier, however, many airliners have
VGs on the wing and even on some parts of
Vortex generators work only when set to the correct angle of 15 degrees as visible on this real HawkGladwin_Layout 1 07/03/18 11.58 Pagina 2